Light-emitting tables for various uses are known, for example, from U.S. Pat. Nos. 5,327,195 and 5,347,342, United States patent application publications 2006/0030026, 2006/0237658, 2007/0069643 and 2008/0308752 and U.S. Pat. No. 8,562,802. United States patent application publication 2006/0030026 discloses that LED light sources for collimating the light emitted by the light sources can be fitted with lens elements. In contrast, in United States patent application publications 2007/069643 and 2006/0237658, without any collimation, only the far field that is homogeneously illuminated by a grid of LED light sources is used as a field for the light-emitting tables, which results in a large installation space for the light-emitting tables.
Moreover, United States patent applications 2006/0104060 and 2005/0190559 disclose the provision of individual LED light sources with a parabolic mirror for collimation of the LED light. Here, the center of the surface of the light-emitting diode chip is located in the focal point or focal line of the parabolic mirror. Laid-open specification DE 103 08 917 A1 furthermore discloses the provision of an LED chip with a conical, spherical or parabolic reflector for collimating the LED light. However, all the stated reflectors for LED light sources are distinguished by the fact that they have a focal point or a caustic for normal incidence of light from infinity and that the optically active surface of the LED chip is located in exactly this focal point or within this caustic.
In the above-mentioned light-emitting tables of the prior art, be it with LED light sources or not, be it with the use of reflector technology for collimation or not, optically diffusing elements, such as for example diffusing plates or diffusing films, are generally used to homogenize the intensity distribution over the light-emitting table field that is to be illuminated. The goal is here generally to limit the homogeneity of the light-emitting table field, that is, the variation of the illumination intensity over the field with respect to the average illumination intensity of the field, to less than 5%. However, this good homogeneity of prior art light-emitting tables is generally achieved by statistically diffusing optical elements, as a result of which the light-emitting table field emits light diffusely in virtually every direction of the half-space located thereabove. However, this broad illumination angle distribution of light-emitting tables of the prior art is unsuitable for many measurement types of coordinates of a workpiece using optical sensors by way of a coordinate measuring machine, in particular for measurements of edges or holes. Consequently, the cost-effective light-emitting tables of the prior art are generally not used, or used only to a very limited extent, for use in metrology.
By contrast, coordinate measuring machines for measuring coordinates of a workpiece to be measured using an optical sensor utilizing what is known as transmitted-light illumination using a complicated illumination optics are known, for example, from U.S. Pat. No. 6,161,940.
Generally, an attempt is made in such coordinate measuring machines for the purpose of increasing resolution to select what is known as the opening angle of the lens used for viewing the workpiece such that it is as large as possible. Accordingly, an associated condenser optics for illuminating the workpiece must provide a tuned maximum illumination angle at the light-emitting table field. Moreover, an attempt is frequently made in these coordinate measuring machines using a variable condenser optics to provide the optimum illumination of the workpiece for the structures to be measured in each case and lens settings. This variability of the condenser optics, however, is at the same time accompanied by a certain complexity in terms of the construction of the condenser optics, which is also associated with correspondingly high costs for producing such a condenser optics.
However, this complicated method of illumination-angle setting in coordinate measuring technology also encounters its limits when it comes to measuring structures having a large extent along the optical axis, since the depth of field decreases as the opening angle increases, and thus structures having a large distance from the light-emitting table field are imaged “without sharpness.” For measuring structures having a large extent along the optical axis perpendicular to the light-emitting table field, such as for example the planes of edges, it therefore makes sense if the illumination light at the light-emitting table field is aligned to be parallel and thus collimated with respect to the optical axis. In this case, structures having a large distance from the light-emitting table field can be measured at the same accuracy as structures having a small distance from the light-emitting table field. In addition, these structures can also be measured in relation to one another, as long as a telecentric lens is used for imaging onto the optical sensor.